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United States Patent |
5,164,947
|
Lukas
,   et al.
|
November 17, 1992
|
Single-frequency, frequency doubled laser
Abstract
A laser is disclosed comprising: a source of optical pumping radiation; a
lasant rod; an input mirror; two quarter-wave plates; polarizing means,
adjacent to a quarter-wave plate, for polarizing said lasant light
radiated from said rod; second harmonic generating means for producing an
output at twice the frequency at which said rod lases; and an output
coupler.
Inventors:
|
Lukas; Gregory J. (Lisle, IL);
Ou; Daniel M. (Glendale Heights, IL);
Anthon; Douglas W. (Wheaton, IL);
Sipes; Donald L. (Lisle, IL)
|
Assignee:
|
Amoco Corporation (Chicago, IL)
|
Appl. No.:
|
662179 |
Filed:
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February 28, 1991 |
Current U.S. Class: |
372/22; 372/21; 372/69; 372/70 |
Intern'l Class: |
H01S 003/10 |
Field of Search: |
372/21,22,69-72
|
References Cited
U.S. Patent Documents
4127827 | Nov., 1978 | Barry | 372/22.
|
4247166 | Jan., 1981 | Yeh | 359/484.
|
4272694 | Jun., 1981 | Jacobs | 359/328.
|
4331891 | May., 1982 | Rizzo | 359/328.
|
4346314 | Aug., 1982 | Craxton | 359/328.
|
4500178 | Feb., 1985 | Yeh | 359/498.
|
4637026 | Jan., 1987 | Liu | 372/22.
|
4809291 | Feb., 1989 | Byer et al. | 372/22.
|
4914664 | Apr., 1990 | Woodward | 372/20.
|
5031182 | Jul., 1991 | Anthon et al. | 372/70.
|
Other References
Heavens & Ditchiburn, "Insight into Optics" Wiley, 1991, p. 89.
|
Primary Examiner: Epps; Georgia Y.
Attorney, Agent or Firm: Gabala; James A., Magidson; William H., Sroka; Frank J.
Claims
We claim:
1. A laser comprising:
a) a lasant material which lases at a predetermined wavelength in response
to optical pumping radiation and which has a front end and a back end;
b) an input mirror for substantially reflecting lasant light towards said
front end of said lasant material;
c) an output coupler for substantially reflecting laser light towards said
input mirror and for passing therethrough at least some laser light at a
harmonic of said predetermined wavelength;
d) means, located between said input mirror and output coupler, for
substantially eliminating spatial hole burning in said lasant material
while producing laser light at said predetermined wavelength;
e) polarizing means, located to receive said light at said predetermined
wavelength from said lasant material; and
f) second harmonic generating means, located to receive said polarized
laser light and having optic axes which are oriented relative to said
polarized light for phase-matching, for converting said polarized laser
light to a harmonic of said predetermined wavelength, said second harmonic
generating means having a length which produces a phase retardation which
is an integral multiple of a half wavelength of said light at said
predetermined wavelength.
2. The laser of claim 1, further including a light emitting semi-conductor
which is used to optically pump said lasant material and which selected
from the group consisting of diode-lasers, superluminescent diodes,
diode-laser arrays and light-emitting diodes.
3. The laser of claim 1, wherein said lasant material is a garnet which is
doped with at least one rare earth selected from the group consisting of
Nd, Ho, Er and Tm.
4. The laser of claim 1, wherein said second harmonic generating means is
made from a material which is selected from the group consisting of KTP,
ADA, ADP, CDA, KDP, MTiO(XO.sub.4), BBO, KNbO.sub.3, LiNbO.sub.3, LBO and
LIO, where "M" is at least one of K, Rb, Tl or NH.sub.4, where "X" is at
least one of P or As and when "M" is NH.sub.4 then "X" is P.
5. The laser of claim 4, wherein said second harmonic generating means is
made from KTP and has optic axes which are positioned at about forty-five
degrees to an optical reference axis which passes through said ends of
said lasant material and which are located in a plane which is generally
perpendicular to said optical reference axis.
6. The laser of claim 1, wherein said length of said second harmonic
generating means is maintained by controlling its temperature to function
as a half-wave plate at the wavelength of said predetermined wavelength.
7. The laser of claim 1, wherein said polarizing means is selected from the
group consisting of a polarizing reflector and an optical surface which
lies in a plane which is at the Brewster angle relative to the light from
said lasant material.
8. The laser of claim 1, wherein said means for substantially eliminating
spatial hole burning comprises two quarter-wave plates, one of said
waveplates being located adjacent to each of said ends of said lasant
material, said waveplates having optic axes which are generally
perpendicular to the axis of the cavity formed by said input mirror and
said output coupler.
9. The laser of claim 2, wherein said light emitting semi-conductor is
close coupled to said back end of said lasant material.
10. The laser of claim 1, further including means to reduce the rearward
going light at said harmonic wavelength.
11. The laser of claim 1, wherein said input mirror is coated to transmit
therethrough to said back end of said lasant material at least some of
said optical pumping radiation.
12. The laser of claim 1, wherein said lasant material is a
non-birefringent crystal.
13. A laser, comprising:
a) a lasant material which lases at a predetermined wavelength in response
to optical pumping radiation from diode-laser means and which has two
opposite ends through which an optical reference axis is located;
b) mirror means, located between one end of said lasant material and said
laser diode means and along said reference axis, for transmitting at least
some of said optical pumping radiation to said one end of said lasant
material and substantially reflecting lasant light at said predetermined
wavelength into said lasant material;
c) polarizing reflector means, adjacent to the other end of said lasant
material, for polarizing said lasant light radiated from said lasant
material, for reflecting substantially said polarized lasant light in a
light path which is at an angle to said optical reference axis, and for
passing therethrough at least some laser light at a harmonic of said
predetermined wavelength;
d) second harmonic generating means, having an effective length which is a
half integral multiple of said predetermined wavelength of said laser
light, for receiving polarized light from said polarizing reflector means
and converting said polarized laser light to said laser light at a
harmonic of said predetermined wavelength, wherein said second harmonic
generating means is characterized by two refractive indices for a given
direction of propagation; and
e) a mirror, located along said light path, for substantially refecting
laser light from said second harmonic generating means in a phased
relationship with said light from said lasant material and towards said
polarizing reflector means.
14. The laser of claim 13, wherein said mirror comprises a reflective
coating on one end of said harmonic generating means.
15. The laser of claim 13, wherein said polarizing reflector means is a
curved mirror.
16. The laser of claim 14, further including means, located along said
reference axis and equivalent to two birefringent quarter-wave plates
having optical axes which are at right angles to said reference axis, for
forming a Lyot filter.
17. A laser comprising:
a) non-birefringent lasant rod means for lasing at a predetermined
wavelength in response to optical pumping radiation and which has two
opposite ends which define an optical reference axis;
b) an input mirror for transmitting at least some of said optical pumping
radiation to one end of said lasant rod means and for substantially
reflecting light at said predetermined wavelength towards said one end of
said lasant rod means;
c) Brewster plate means, located along said optical reference axis, for
polarizing said light at said predetermined wavelength;
d) birefringent second harmonic generating means, located along said
optical reference axis, for receiving said polarized light and converting
said polarized laser light to a harmonic of said predetermined wavelength;
e) two quarter-wave plates which are located adjacent to said opposite ends
of said lasant rod means and which have optic axes which are aligned
relative to each other to function, with said Brewster plate means, as one
Lyot filter;
f) means for maintaining the temperature of said second harmonic generating
means to function, with said Brewster plate, as a second Lyot filter; and
g) an output mirror, located along said optical reference axis, for
substantially reflecting laser light at said predetermined wavelength
towards said Brewster plate means and for transmitting therethrough at
least some of said light at said harmonic of said predetermined
wavelength.
18. The laser of claim 17, wherein said second harmonic generating means
has optical axes which are aligned for phase-matching said harmonic
generating means to said polarized light.
19. A single frequency laser, comprising:
a non-birefringent lasant material which is adapted to lase at a
fundamental wavelength in an optical cavity in response to a source of
optical pumping radiation;
spatial hole burning elimination means, located adjacent to at least one
end of said lasant material, for substantially eliminating spatial hole
burning in said cavity; and
Lyot filter means, in said cavity, for monochromatically polarizing lasant
light radiated from said lasant material and for converting at least some
of said polarized lasant light to light at substantially a harmonic
wavelength.
20. The laser of claim 19, wherein said spatial hole burning elimination
means comprises:
one quarter-wave plate which is located at one end of said lasant material
and which has a fast axis; and
an essentially identical quarter-wave plate which is located at the
opposite end of said lasant material and which has a fast axis which is at
an angle to said fast axis of said one quarter-wave plate.
21. The laser of claim 20, wherein said one quarter-wave plate carries an
input mirror for one end of said cavity.
22. The laser of claim 21, wherein the opposite end of said cavity
comprises an output coupler for substantially reflecting laser light
towards said input mirror.
23. The laser of claim 22, wherein said Lyot filter means is adjacent to
the opposite end of said lasant material.
24. The laser of claim 20, wherein said Lyot filter means comprises:
polarizing means for polarizing light along a direction of polarization;
and
a phase-matched, birefringent, frequently-doubling material whose axes are
positioned at acute angles to said direction of polarization established
by said polarizing means, said fast axes of said quarter-wave plates lying
on each side of said direction of polarization, said birefringent material
having an effective length equal to an integral multiple of half the
wavelength of said light at said fundamental wavelength.
25. The laser of claim 24, wherein said birefringent material is
temperature tunable to function as an integral multiple of half the
wavelength of said light at said fundamental wavelength.
26. The laser of claim 25, wherein said source is a diode-laser; and
further including: means for controlling the temperature of said
birefringent material; and means for controlling the temperature of said
diode-laser.
27. The laser of claim 26, wherein said temperature of said diode-laser is
separately controllable from the temperature of said birefringent
material.
28. The laser of claim 24, wherein said polarizing means is a Brewster
plate.
29. A method of producing green or blue light at essentially a single
frequency, comprising the steps of:
a) locating a lasant rod in an optical cavity which is formed by two
mirrors and which defines a reference axis;
b) pumping one end of said rod with diode-laser means to produce light at
an infrared or near infrared wavelength;
c) polarizing said light from said rod along a direction of polarization;
d) using a frequency doubling crystal to convert said polarized light to
light whose wavelength is about half the wavelength of said light from
said rod, said crystal material having optic axes which are arranged for
phase-matching relative to said direction of polarization; and
e) maintaining said crystal at a temperature such that it produces a phase
shift which is an integral multiple of one half the wavelength of said
polarized light from said rod.
30. The method of claim 29, further including the step of eliminating
spatial hole burning in said rod by locating a quarter-wave plate adjacent
to each end of said rod, and by positioning both plates at right angles to
said reference axis.
31. The method of claim 29, wherein step (c) is performed by using a
transparent plate which lies in a plane which is at the Brewster angle
relative to said reference axis.
32. The method of claim 29, wherein said lasant rod is made from Nd:YAG;
and wherein said crystal is made from KTP.
Description
Technical Field
This invention relates to the general subject of lasers, and, in
particular, to the subject of solid-state, diode-laser pumped frequency
doubled lasers.
BACKGROUND OF THE INVENTION
Intracavity doubled Nd:YAG lasers were proposed as sources of green light
more than 20 years ago, and many such devices have been built and analyzed
in the ensuing years. Typical devices consisted of a Nd:YAG rod, a
Brewster polarizer and a Type-I phase-matched crystal, such as Ba.sub.2
NaNb.sub.5 O.sub.15 or LiIO.sub.3. Several examples of this type of device
are shown in the book by Koechner, Solid State Laser Engineering,
Springer-Verlag, 2nd edition, 1988. In general, it was observed that these
devices were much less stable with the non-linear crystal in the cavity
than they were without it. Several tentative explanations involving
mode-beating or thermal effects were suggested, but no definitive studies
were carried out. It was often thought that the non-linear crystal was
simply a non-linear amplifier for fluctuations already present in the
undoubled laser. Stability was not the only problem with these devices;
crystal damage and other materials problems tended to limit the
performance of the devices.
Interest in intracavity doubled lasers was renewed in the 1980's when new
non-linear materials and diode-laser pumping techniques became available.
One new non-linear material was KTiOPO.sub.4, potassium titanyl phosphate
or KTP, a highly non-linear material which was free from many of the
mechanical, thermal and optical problems which had plagued earlier
materials. Phase-matching in KTP is Type-II, so the simple Brewster plate
polarizers used with earlier materials were not adequate.
Using Type-II non-linear crystals, such as KTP, for intra-cavity second
harmonic generation (SHG) introduces a variety of polarization related
problems. Placing a birefringent crystal in an unpolarized laser cavity
often produces undesirable effects because the crystal axis will define
two orthogonal polarizations that will, in general, differ in both their
optical path lengths and losses. The path length difference leads to two
weakly coupled sets of resonant cavity frequencies which often give rise
to erractic mode-hopping behavior and output noise. Furthermore, any
differenes in the relative losses for each polarization tend to result in
a laser output which is polarized along one axis of the crystal. Since
radiation polarized along two crystal axes is required for Type-II
doubling, output radiation polarized only one axis would prevent or at
least degrade the efficiency of the SHG process. Retardation plates have
been used to control the polarizations inside the cavity. The issue of
noise was not addressed. Typical examples are found in the following U.S.
Pat. Nos.: 4,413,342 to Cohen, et al.; 4,127,827 and 3,975,693 to Barry et
al.; 4,617,666; 4,637,026; 4,048,515; and 4,618,957 to Liu.
Baer appears to have been the first to have built a diode-laser pumped
Nd:YAG laser which was intracavity doubled with KTP. See for example, U.S.
Pat. Nos.: 4,653,056; 4,656,635; 4,701,929; 4,756,003; and 4,872,177. An
early cavity used by Baer consisted of an end-pumped Nd:YAG rod, a KTP
crystal and a curved reflector, and had no polarization controlling
elements. Baer reported the following results: (1) when the laser was
operated without an intracavity etalon, it exhibited optical noise having
a frequency in the range from about 10 kilohertz to multiples of 100
kilohertz; (2) when an etalon was added to reduce the number of
oscillating modes to two, well-defined oscillations in optical power were
observed; and (3) when the laser was forced to run in a single mode with
an etalon, the output power was stable, but the laser produced very little
green output. Baer interpreted his results in terms of a rate equation
model which included both sum generation and cross saturation effects.
Baer believed that the laser amplitude fluctuations occurred because the
system has two non-linear feedback mechanisms operating on two differentt
timescales. He concluded that the oscillations were a fundamental barrier
to successful multimode operation of intracavity doubled lasers.
Later designs by Baer added a Brewster plate polarizer oriented at
45.degree. from the axis of the KTP to provide equal power in the two
crystal polarizations. This design suffered from the fact that, in
general, a Brewster plate and a birefringent crystal cannot be combined in
a low-loss optical cavity. The linear polarization passed by the Brewster
plate will be transformed by the KTP into an elliptical polarization that
will experience a significant loss upon passing through the Brewster
plate. Only in the special case (not described or discussed by Baer) when
the KTP functions as a half-integral waveplate, will the cavity losses be
low. Because KTP is strongly birefringent, having temperature-dependent
refractive indices, a typical few millimeter long, doubling crystal of KTP
will act as a temperature-variable, multiple-order retardation plate. In
general, for low-loss eigenmodes to exist in a laser cavity containing a
Brewster plate and a birefringent element, the birefringent element must
be a full-or half-wave plate. Thus, success in producing a low loss
optical cavity at a given wavelength is critically dependent upon rigid
control of the crystal length and the cavity temperature. A sensitive
inter-relationship exists between crystal length, cavity temperature and
polarization losses.
Others have also attempted to make a solid-state laser which uses
non-linear crystal or lasant material to produce green light from infrared
light using the principles of second harmonic generation. The following
U.S. patents are illustrative of the many practitioners who have attempted
to make a practical apparatus: U.S. Pat. Nos. 3,624,549 to Geusic et al.;
3,750,670 to Palanos et al.; 3,619,637 to Godo et al.; and 4,856,006 to
Yano et al.
More recently, Anthon et al. disclosed an intracavity frequency doubled
laser (U.S. Pat. Nos. 4,933,947 and assigned to AMOCO Corporation) having
improved amplitude stability. This was achieved by substantially
eliminating spatial hole burning in the lasant material and by maintaining
the optical cavity of the laser at a temperature which results in
substantially noise-free generation of optical radiation.
Despite what appears to be a fairly complete, general understanding of the
theory of the frequency doubling process, a reliable, solid-state,
diode-laser-pumped, frequency-doubled laser has yet to find complete
acceptance in the market place. Heretofore such lasers have been plagued
with a variety of problems. These problems have included: a variation in
power output during start-up; output powers which vary significantly with
changes in temperature and over time; non-repeatable output power with a
variation in cavity temperature; multiplide (e.g. two or three spectral
modes running simultaneously; differing polarizations in the spectral
modes without any consistent relationship between them; an infrared (IR)
polarization which was not defined; spectral modes and output powers which
change when the laser is tapped or slightly vibrated; and laser operation
(i.e., output power and spectral modes) which seem to be unduly sensitive
to normally occurring changes in the characteristics of the pumping
diode-laser.
Clearly a reliable and dependable, single-frequency, frequency-doubled
laser would be welcomed by the photonics industry. More importantly, if a
solid-state, diode-laser source of infrared optical pumping radiation is
used, a miniature source of visible green light can be obtained.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a reliable source of
essentially single-frequency, frequency-doubled laser light.
Another object of the invention is to provide a diode-laser-pumped,
solid-state laser which produces a stable and reliable source of green
light.
Yet another object of the invention is to provide a stable green laser
which uses a laser diode, a rare earth doped crystal, and a frequency
doubling material in a common cavity.
Another object of the invention is to provide a stable and reliable
solid-state, diode-laser-pumped laser which uses Nd:YAG and KTP, which
does not have the problems resulting from spatial hole burning, and which
has a single-frequency spectral mode for an output.
Still another object of the invention is to provide a frequency-doubled
laser system having a cavity which ensures doubling and avoids a polarized
mode of operation wherein the oscillating modes compete.
Another object of the invention is to combine Lyot filter principles and
second harmonic generating principles to produce a single-frequency,
frequency-doubled laser.
In accordance with the present invention a laser is disclosed comprising a
lasant material which is adapted to lase at a predetermined wavelength in
response to optical pumping radiation and which has a front end and a back
end; an input mirror for substantially reflecting lasant light towards
said back end of said lasant material; an output coupler for substantially
reflecting laser light towards said input mirror and for passing
therethrough at least some laser light at a harmonic of said predetermined
wavelength; means, located between said input mirror and said output
coupler, for substantially eliminating spatial hole burning in said lasant
material while producing laser light at said predetermined wavelength;
polarizing means, located to receive said light at said predetermined
wavelength from said lasant material, for polarizing said lasant light
from said lasant material; and second harmonic generating means, located
to receive said polarized laser light and having optical axes which are
orientated relative to said polarized light for phase-matching, for
converting said polarized laser light to a harmonic of said predetermined
wavelength.
The laser system just described, when KTP is used as the second harmonic
generating means and when pumped with near infrared light, functions as a
stable source of green light. The single frequency laser of the present
invention has been found to be a precise source of power, since it runs in
a well-defined, single spectral mode with consistent polarization.
Moreover, the power is high, spatial hole burning is eliminated, the
output power variations due to changes in cavity temperature are
repeatable and the IR polarization is well defined regardless of cavity
temperature and mode. In addition, the mode distribution does not change
when the laser is slightly vibrated, struck or if the pump mode changes.
Numerous other advantages and features of the present invention will become
readily apparent from the following detailed description of the invention,
the embodiments described therein, from the claims, and from the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the single frequency green laser that is
the subject of the present invention;
FIG. 2 is a schematic diagram of another embodiment of the invention of
FIG. 1; and
FIG. 3 is a schematic diagram of a variation of the embodiments shown in
FIG. 2.
DETAILED DESCRIPTION
While this invention is susceptible of embodiment in many different forms,
there is shown in the drawings, and will herein be described in detail, at
least three specific embodiments of the invention. It should be
understood, however, that the present disclosure is to be considered an
exemplification of the principles of the invention and is not intended to
limit the invention to the specific embodiments illustrated.
Turning to FIG. 1, there is illustrated a single frequency green laser 10
which comprises: a source 12 of optical pumping radiation, a lasant
material 14, an input mirror 16, an output coupler 18, two quarter-wave
plates 20 and 22, a polarizing element 24, and a non-linear, optical,
harmonic-generating material or element 26 (i.e., alternatively referred
to as a "frequency doubler"). A temperature control 28 is used to control
a thermo-electric cooler 30 for the source 12 and heaters or coolers 32
and 33 for the laser cavity.
The source 12 provides optical pumping radiation to the lasant material 14.
A focusing means or lens 11 (e.g., an optical element having a gradient
refractive index or GRIN, a ball lens, an aspheric lens, a combination of
lenses, etc.) can be used to focus the output of the source 12 onto the
lasant material 14. This focusing results in a high pumping intensity and
an associated high photon to photon conversion efficiency in the lasant
material 14. Any number of combinations of sources and lasant materials
can be used.
Preferably, the source 12 is a light emitting semi-conductor, such as a
diode-laser or diode-laser array, and the laser material 14 is a
non-birefringent crystal, such as a garnet doped with a rare-earth, active
material (e.g., Nd:YAG), or a crystal that includes a rare-earth, active
material which is a stoichiometric component of the lasant host material
(e.g., lithium neodymium tetraphosphate (LNP) or neodymium pentaphosphate
(NPP).
If desired, the output facet of the semiconductor light source 12 can be
placed in a close coupled or in butt-coupled relationship to input face of
the lasant material 14 without the use of a focusing means 11. As used
herein, "butt-coupled" is defined to mean a coupling which is sufficiently
close such that a divergent beam of optical pumping radiation emanating
from semiconductor light source 12 will optically pump a mode volume
within a lasant material 14 with a sufficiently small transverse
cross-sectional area so as to support essentially only single transverse
mode laser operation (i.e., TEM.sub.00 mode operation) in the lasant
material 14.
Highly suitable lasant materials 14 for butt-coupled operation include
neodymium-doped YAG or Nd:YAG, gadolinium gallium garnet (Gd.sub.3
Ga.sub.5 O.sub.12) or GGG, and gadolinium scandium gallium garnet
(Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12) or GSGG, and especially LNP or NPP.
By way of specific example, neodymium-doped YAG is a highly suitable
lasant material for use in combination with an optical pumping means which
produces light having a wavelength of about 800 nm. When pumped with light
of this wavelength, neodymium-doped YAG can emit light having a wavelength
of approximately 1064 nm.
The precise geometric shape of lasant material 14 can vary widely. For
example, lasant material can be rod-shaped, or rhombohedral in shape if
desired. If desired, an end-pumped fiber of lasant material can be used.
Highly suitable fibers for this purpose include, but are not limited to,
glass optical fibers which are doped with ions of a rare-earth metal such
as neodymium. The length of such a fiber is easily adjusted to result in
absorption of essentially all of the optical pumping radiation. If a very
long fiber is required, it can be coiled, on a spool for example, in order
to minimize the overall length of the laser of this invention.
A highly suitable source 12 of optical pumping radiation consists of a
gallium aluminum arsenide laser diode array, emitting light having a
wavelength of about 800 nm, which is attached to heat sink. Such laser
diodes are well known to those skilled in the art and may be obtained from
a variety of suppliers (e.g., Spectra-Diode Laboratories, SONY, Laser
Diode Inc., Siemens, etc.). The heat sink can be passive in character.
However, the heat sink can also comprise a thermoelectric cooler 30 to
help maintain laser diode array 12 at a constant temperature and thereby
ensure optimal operation of laser diode array at a constant wavelength.
The temperature of a laser diode source 12 can be regulated by means of
control electronics 28. Separate controls for temperature regulation of
the source 12 and the pumped laser cavity can be used. It will also be
appreciated, of course, that during operation the optical pumping means or
source 12 will be attached to a suitable power supply. Electrical leads
from laser diode array which are directed to a power supply are not
illustrated in the drawings for clarity.
A laser cavity having a longitudinal extending axis 34 is formed by the
input mirror 16 and the output coupler or mirror 18. Both mirrors are
highly (e.g., 99% or more) reflective (HR) at the wavelength (i.e., the
fundamental or .lambda..sub.F) of the lasant material rod 14 (e.g., 1064
nm for Nd:YAG). The input mirror 16 is coated so as to transmit the light
(e.g., 800 nm) the source 12 (e.g., highly transmissive (HT) at about 85%
or more) and to be highly reflective at the wavelength of the fundamental.
The output coupler 18 is a mirror which is coated to be HT at the harmonic
of the wavelength of light emitted by the lasant material or rod 14 (e.g.,
532 nm (green) for Nd: YAG lasing at 1064 nm). The output mirror 18 and
input mirror 16 are conventional in character and, for example, can
comprise suitable conventional coatings on appropriate substrates.
Light from the lasant material or rod 14 interacts with the non-linear
optical material 26 to double the frequency of the light from the lasant
material. Materials having non-linear optical properties are well-known.
For example U.S. Pat. No. 3,949,323 issued to Bierlein et al. discloses
that non-linear optical properties are possessed by materials having the
formula MTiO(XO.sub.4) where "M" is at least one of K, Rb, Ti and NH.sub.4
; and "X" is at least one of P or As, except when NH.sub.4 is present,
then "X" is only P. This generic formula includes potassium titanyl
phosphate (KTP) or KTiOPO.sub.4, a particularly useful non-linear
material.
Preferably the frequency doubling material 26 is KTP. KTP has one of the
highest non-linear optical coefficients. KTP is a biaxial material having
axes which are preferably arranged for Type-II phase-matching (e.g.,
having its Z-axis perpendicular to the reference axis 34 of the cavity and
at about 45.degree. to one side of a plane (i.e., a plane coincident with
the plane of drawings and the direction of polarization established by the
polarizing element 24) which lies along the reference axis).
Non-linear optical material, such as KTP, have the ability of converting
light at predetermined of fundamental wavelength into light at a harmonic
of that light (i.e., light at frequency .omega. is converted to light at
the second harmonic 2 .omega. or near-infrared light at a wavelength of
1064 nm is converted to green light at a wavelength of 532 nm). Other
non-linear optical materials which are suitable for frequency doubling
include: potassium dihydrogen phosphate (KDP) or KH.sub.2 PO.sub.4 ;
ammonium dihydrogen phosphate (ADP) or NH.sub.4 H.sub.2 PO.sub.4 ;
ammonium dihydrogen arsenate (ADA) or NH.sub.4 H.sub.2 AsO.sub.4 ; cesium
dideuterium arsenate (CDA) or CsH.sub.2 AsO.sub.4 ; beta-barium-borate
(BBO) or .uparw.-BaB.sub.2 O.sub.4 ; lithium triborate (LBO) or LiB.sub.3
O.sub.5 ; as well as KTiOAsO.sub.4, lithium idodate (LIO) or LiIO.sub.3,
LiNbO.sub.3, KNbO.sub.3, HIO.sub.3, KB.sub.5 O.sub.8 -4H.sub.2 O,
KLiNbO.sub.3, and organic materials including urea. A review of the
non-linear optical properties of a number of different uniaxial crystals
has been published in Sov. J. Quantum Electron, Vol. 7, No. 1, Jan. 1977,
pp. 1-13. Non-linear optical materials have also been reviewed by S. Singh
in the CRC Handbook of Laser Science and Technology, Vol III, M.J. Weber,
Ed., CRC Press, Inc., Boca Raton. Fla., 1986, pp. 3-228.
The conversion of optical radiation of one frequency to optical radiation
of another frequency through interaction with a non-linear optical
material is well-known and has been extensively studied. Examples of such
conversion include harmonic generation, optical mixing and parametric
oscillation. Second-harmonic generation or "frequency doubling" is perhaps
the most common and important example of non-linear optics wherein part of
the energy of an optical wave of angular frequency .omega. propagating
through a non-linear optical crystal is coverted to energy of a wave of
angular frequency 2 .omega.. Second-harmonic generation has been reviewed
by A. Yariv in Quantum Electronics, Second Ed., John Wiley & Sons, New
York, 1975 at pages 407-434 and by W. Koechner in Solid State Laser
Engineering, Springer-Verlag, N.Y., 1976 at pages 491-524.
Electromagnetic waves having a frequency in the optical range and
propagating through a non-linear crystal are believed to induce
polarization waves which have frequencies equal to the sum and difference
of those of the exciting waves. Such a polarization wave can transfer
energy to an electromagnetic wave of the same frequency. Those skilled in
the art know that the efficiency of energy transfer from a polarization
wave to the corresponding electromagnetic wave is a function of: (a) the
magnitude of the second order polarizability tensor of the optical
material (since this tensor element determines the amplitude of the
polarization wave); and (b) the distance over which the polarization wave
and the radiated electronmagnetic wave can remain sufficiently in phase,
or "phase-matched" for the non-linear conversion process.
A conventional method for achieving such phase-matching in a non-linear
optical material utilizes the fact that dispersion (the change of
refractive index with frequency) can be offset by using the natural
birefringence of uniaxial) or biaxial crystals. Such crystals have two
refractive indices for a given direction of propagation which correspond
to the two allowed, orthogonally-polarized propagation modes. Accordingly,
by an appropriate choice of polarization, direction of propagation and
crystal axes orientation, it is often possible to achieve phase-matching
in a birefringent non-linear optical crystal. The term "phase-match axis,"
as used herein, refers to a line or direction through a non-linear optical
crystal along which the substantially phase-matched conversion of a stated
input radiation into a stated output radiation is permitted for at least
certain polarization of said input radiation. Type-I phase-matching
requires that the incident waves interacting in the non-linear material
have the same polarization. Type-II phase-matching requires that the
incident waves interacting in the non-linear optical material have
orthogonal polarizations.
KTP is a frequency doubling material that can be Type-II phase matched.
Such a material 26 is preferably temperature tuned so that it has an
effective length of an integral multiple of half the wavelength of the
fundamental (e.g.,
##EQU1##
etc., where .lambda..sub.F is the wavelength at which the lasant rod 14
lases and .lambda..sub.2 =.lambda..sub.F /2, where .lambda..sub.2 is the
wavelength of the harmonic). A heater 32 and conventional control
electronics 28 can be used for this purpose. In a short cavity,
temperature regulation of the laser diode source 12 can affect the
temperature of the frequency doubling, non-linear optical material 26.
Preferably, a pre-established temperature gradient is detected and
maintained. A heating element 33 adjacent to the lasant rod 14 and another
element 32 adjacent to the frequency doubler 26 can be used to establish
and maintain a desired temperature gradient. Judicious selection and
location of temperature sensors will minimize temeprature feedback and
cross-talk between the source cooler 30 and the cavity gradient heaters 33
and 32. Alternatively, the frequency doubling material 26 can be housed to
insulate it from the source 12. By keeping the frequency doubler 26 at
this preferred length, polarized light at the fundamental wavelength will
under go a phase shift of an integral multiple of 180.degree. each time it
passes through the frequency doubler.
The two quarter-wave plates 20 and 22 function primarily as a means for
substantially eliminating spatial hole burning in lasant material by
causing circular polarization of the cavity radiation and thereby creating
a "twisted mode" optical cavity. The twisted mode technique for producing
an axially uniform energy density in a laser cavity is described by V.
Evtuhov et al., Appl. Optics, Vol 4. No. 1, pp. 142-143 (1965). Also see
Draegert, "Efficient Single-Longitudinal-Mode Nd:YAG Laser," IEEE J.
Quant. El., QE-8, 235 (1972).
Any conventional means for substantially eliminating spatial hole burning
in the lasant material can be used in the practice of this invention. For
example, spatial hole burning can be eliminated through the use of a
traveling wave, ring-like optical cavity, by mechanical motion, or by
electro-optic phase modulation. Here, the quarter-wave plate 22 which is
located next to the polarizing element 24 is oriented with an axis (e.g.,
its fast axis) at about b 45.degree. from a plane containing the direction
of polarization established by the polarizing element 24 along the
reference axis 34. This same quarter-wave plate 22 is also aligned to the
otpical axis of the frequency doubler 26.
Preferably the waveplates are identical, and corresponding axes of the
waveplates are "crossed" or arranged with corresponding axes at right
angles to each other and the reference axis 34 (e.g., fast axis of
waveplate 22 is perpendicular to the fast axis of waveplate 20). Quartz
waveplates 20 and 22 having a thickness of 1.01 mm can be used. The
quarter-wave plates 20 and 22 are located adjacent to opposite ends of the
lasant rod 14. The result is standing wave pattern in the cavity which is
linearly polarized at the cavity end mirrors 16 and 18. The mode is
circularly polarized in the laser rod 14;, this gives a standing wave
where the electric field vector rotates through the gain medium or laser
rod, and where there are no standing wave nodes within the gain medium.
The function of the input mirror 16 can be obtained by coating one
waveplate 20 with suitable reflective coatings (e.g., anti-reflection (AR)
at about 800 nm and HR at approximately 1064 nm on one side and AR at 1064
and 800 nm on the other side). This reduces the number of components and
the total cost.
The last component of the laser 10 is a polarizing element 24. Preferably,
the polarizing element 24 is a Brewster plate whose plane is at the
plate's Brewster angle to the reference axis 34. The polarizing element 24
establishes a direction of polarization within the laser cavity which,
according to the orientation of FIG. 1, is in the plane of the drawings.
The two quarter-wave plates 20 and 22, together with a non-birefringent
lasant material (e.g., Nd: YAG) form a field of circular polarized light
which are summed together in going from one end to the other end of the
resonant laser cavity. The polarization can also be achieved by means of
coatings on mirrors, a dielectric polarizer, or other suitable
polarization means. A Brewster angled surface at one end of the lasant rod
14 can be used if quarter-wave plates are not needed to control spatial
hole burning.
The combination of the polarizing element 24 and a birefringent frequency
doubling material 26, having an effective length which is an integral
multiple of half the wavelength of the fundamental, functions in a manner
which is similar to a Lyot filter (i.e., a Lyot-Ohman filter) for laser
radiation reflecting back and forth within the cavity. A Lyot filter is a
monochromatic polarizer. Since spatial hole burning is controlled by the
two quarter-wave plates 20 and 22, and uniform intensity between the two
quarter-wave plates is achieved by the polarizing element 24, the output
light from this cavity is essentially single frequency.
To limit the number of oscillating spatial modes (i.e., maintain the
TEM.sub.00 mode) an aperture disk "D" can be inserted between the laser
rod 14 and the frequency doubling element 26. In one embodiment, where the
frequency doubler 26 was KTP having a cross section of about 1.5 mm.sup.2,
an aperture of approximate 0.03 inches in diameter was used. Those skilled
in the art will understand that the aperture disk may be located anywhere
in the cavity and that its size is a function of pump cross section,
mirror radius and cavity length.
The components of the present invention may be assembled in any one of a
number of different ways. Assembly is facilitated by building the laser
from two sub-assemblies, in particular, an upper sub-assembly comprising:
the output mirror 18, the frequency doubler 26, an aperture disk "D" (if
used) and the polarizing element 24; and a lower assembly comprising: a
GRIN lens, two quarter-wave plates 20 and 22, and the laser rod 14. More
specifically, each sub-assembly is formed by mounting the components in
disk-like holders, stacking the holders, and locking the holders in place.
In forming the upper sub-assembly, the axes of the frequency doubler 26
are preferably at about 45.degree. to the direction of polarization
established by the polarizing element 24. After each subassembly is
formed, the two sub-assemblies are stacked and locked together, making
sure that the principal direction or axes of the KTP or frequency doubler
26 of the upper sub-assembly are aligned as closely as possible (e.g.,
within one degree of arc or better) to the axes of the quarter-wave plate
22 (i.e., in the lower sub-assembly) which is located adjacent to the
polarizing element 24.
Another embodiment of the invention is shown in FIG. 2. All of the
components of that laser 40 are the same with the exception that a
polarizing reflector or bending mirror 42 is used as an output coupler and
a dual band mirror 44 is used to form an L-shaped cavity. The polarizing
reflector 42 is HR at the fundamental wavelength for one polarization and
transmissive at the harmonic wavelength; it serves to replace the
polarizing element 24 shown in FIG. 1. The polarizing reflector 42
reflects light at the incoming fundamental in a path 46 which is at right
angles to the path 48 of the incoming radiation. This is done to maximize
the green output while avoiding the destructive interference effects which
can occur with reflected second harmonic light.
The intensity of the green output at the polarizing reflector 42 will
depend on the relative phases of the 532 nm harmonic beams reflected off
the dual band mirror 44 at one end of the frequency doubler 26, and the
532 nm beam generated on the second pass through the frequency doubler. If
they are optimally phased, the output will be four times that generated in
a single pass. If they are out of phase, the two green outputs will
cancel, leading to a reduced green output. The phase of the reflected beam
will depend upon the design of the dual band mirror 44 as well as the
distance between the dual band mirror and the frequency doubler 26. The
latter effect is due to the dispersion of air. By placing the dual band
mirror 44 fairly close to one end of the frequency doubler 26, or applying
the reflective coating directly to one end of the frequency doubler, the
distance effect can be minimized. Phase-preserving coatings are known to
those skilled in the art and need not be further discussed.
The Lyot filter, comprising the frequency doubler 26 and the polarizing
reflector 42 (or other suitable polarizing element 24 of FIG. 1), is a
strongly wavelength-selective element. The transmission calculated for
these two elements is such that the laser could be operated successfully
in a single frequency regime without the elimination of spatial hole
burning; thus, the quarter-wave plates 20 and 22 may not be needed. The
elimination of the quarter-wave plates 20 and 22 allows the design shown
in FIG.'s 1 and 2 to be implemented using fewer components. One potential
weakness of the Lyot filter is its relatively narrow, free-spectral range;
for example, it is possible to have low-loss modes at both 1064 nm and
1061 nm. Nevertheless, this difficulty can be resolved in at least two
ways:
1) The quarterwave-plates 20 and 22 can serve another function besides the
elimination of spatial hole burning. If they are of different thicknesses,
or if they are oriented with their optical axis aligned, then they
constitute a second birefringent element in the cavity. Moreover, since
they are at the opposite end of the cavity from the frequency doubler 26,
they form a second elementary Lyot filter. By suitable choice of lengths
or thicknesses of the birefringent material, the free spectral range of
this second Lyot filter can be made much larger than that of the Lyot
filter at the other end of the cavity. This results in a situation where
the KTP Lyot filter has the effect of selecting a specific mode at which
the laser operates, while the quarterwave-plate Lyot filter functions to
prevent oscillation of some further removed undesirable mode (e.g., ones
at 1061 nm and 1074 nm). Thus, in this configuration, and where there is
only one low-loss mode available, there is probably no need to directly or
specifically eliminate spatial hole burning. A similar effect can be
obtained if the two quarter-wave plate combination is replaced with a
single full-wave or half-wave plate.
2) A somewhat similar effect can be achieved by locating a birefringent
lasant material oriented at 45.degree. to the polarizer axis. This forms a
second Lyot filter based on the length of the gain medium. A second Lyot
filter is likely to provide a free spectral range similar to that of using
the KTP as a frequency doubler. However, by using the vernier principle,
it should be possible to get the two filters to coincide at the desired
wavelength and suppress all other lasing under the gain curve. This would
then have a significant effect on spatial hole burning, since the standing
waves and the two polarizations will have different wavelengths. While
this may not be as efficient as two quarter-wave plates for reducing the
residual gain, it should reduce it substantially and may be enough by
itself to achieve the desired operating conditions.
FIG. 3 shows a cavity design 50 which incorporates some of these concepts.
The axes of the KTP frequency doubler 26 are oriented at 45.degree. to the
polarizing reflector 52 to form a Lyot filter. Coatings 44 to form a
phased reflector are located directly at the far end of the frequency
doubler 26, wherein there is no ambiguity in the path length. If the KTP
frequency doubler 26 has parallel end faces, both faces of the KTP
frequency doubler are perpendicular to the beam, thereby minimizing
losses. Here, the lasant rod of Nd:YAG 14 is of a plano-plano
configuration, and has one end coated such that, with the coating 44 on
the KTP frequency doubler, a laser cavity is formed. The parallelism of
the rod 14 forces the perpendicularity of the AR coated face. Unlike FIG.
2, this cavity design 50 uses a curved polarizing reflector 52. One
disadvantage of using a curved polarizing reflector or polarizing mirror
is that it produces an elliptical beam. Nevertheless, it gives a simple
self-aligning cavity design. The polarizing reflector can be replaced with
a flat mirror if the input face of the Nd:YAG rod 14 is curved
appropriately. A half-wave or full-wave plate 54 is shown in phantom and
is used in the event that a second Lyot filter proves to be necessary.
Thus, the two quarter-wave plates (used in FIG. 1 or FIG. 2) are replaced
with a single half-wave or full-wave plate 54 in the design of FIG. 3.
From the foregoing description, it will be observed that numerous
variations, alternatives and modifications will be apparent to those
skilled in the art. Accordingly, this description is to be construed as
illustrative only and is for the purpose of teaching those skilled in the
art the manner of carrying out the invention. Various changes may be made,
materials substituted and features of the invention may be utilized. For
example, by using means (e.g., at location "D" in FIG. 1) to control the
rearward going green light (e.g., a mirror which is located between the
polarizing means 24 and the frequency doubler 26 which is coated on one
face for AR at 1064 nm and which is coated on the opposite face for AR at
1064 nm and HR at about 532 nm), the given polarization can be made to be
directionally dependent and the power of the laser quadrupled over that of
a single pass device. In addition, an electro-optic effect can be used
with the Lyot filter to fine tune the laser to run with the least losses
and at the center of the lasant material gain curve. Moreover, instead of
using a separate lasant material and a frequency doubler, a self-doubling
lasant material (e.g., Tm:LiNbO.sub.3 or Nd:YAB) in combination with Lyot
filtering means is suggested. Finally, the present invention can be used
to produce blue light, as well as green light; the same principles apply.
Thus, it will be appreciated that various modifications, alternatives,
variations, etc., may be made without departing from the spirit and scope
of the invention as defined in the appended claim. It is, of course,
intended to cover by the appended claims all such modifications involved
within the scope of the claims.
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